Experimental evidence for the essential identity of the selective and normal photo-electric effects

Millikan, R. A. and Souder, W. H.
(1916)
Experimental evidence for the essential identity of the selective and normal photo-electric effects.
Proceedings of the National Academy of Sciences of the United States of America, 2
(1).
pp. 19-24.
ISSN 0027-8424.
http://resolver.caltech.edu/CaltechAUTHORS:MILpnas16a

Abstract

In the fall of 1913, while studying the photo-electric properties of freshly cut surfaces of the alkali metals in extreme vacua, we observed that immediately after first cutting, the fresh surface of sodium showed very large photo-sensitiveness when tested with monochromatic light of wave length 5461 A., even when the vacuum was of the order 10(-6) mm. as measured by a McLeod gauge. But after several weeks of experimenting and many cuttings a condition was reached in which a freshly cut surface was completely insensitive when illuminated with this wave length. The lost sensitiveness reappeared, however, in the course of not more than two minutes after cutting, and grew rapidly to a very large value in fifteen or twenty minutes. When the gas pressure was of the order of 0.01 mm. the same phenomenon occurred but the rise to a maximum value was less rapid. From these results we began to surmise that photo-electric currents must be due to the influence of some active gas, which diffused from the walls to the metal and whose action upon the surface was retarded by the presence of an inert gas.